A new analytic formula for the neoclassical bootstrap current has been developed based on NEO (https://aip.scitation.org/doi/10.1063/5.0012664.) It was previously known that the Sauter model for the bootstrap current is inaccurate at high collisionality and in the presence of impurities. The new formula has the same analytic form as the original Sauter model, using only three fitting parameters: fraction of trapped particles, the collisionality, and the effective charge number. The new model shows better agreement with NEO in the H-mode pedestal. The new bootstrap current model has been implemented in NEO and is available through the VGEN bootstrap current generator for use in integrated modeling.

Predicting the incident heat flux in future tokamaks, like ITER, is crucial to avoid divertor damage. Non-axisymmetric shaping of plasma facing components (PFCs) is a new technique to avoid issues such as melting of leading edges. Like applied 3D perturbation fields, commonly used for plasma control, this creates toroidal asymmetries in the divertor heat flux patterns that can be observed on the divertor plates (see Highlight from https://fusion.gat.com/theory/Weekly_Highlight_July_16.2021). On NSTX-U it has been proposed to use a sawtooth-like profile in the toroidal direction for divertor tiles to shadow leading edges of neighboring tiles from incident heat flux. A new toolset, called HEAT, was developed to model the effect of 3D shaped PFCs. The tool uses a CAD model of the inner wall with all gaps, and traces heat flux from an axisymmetric EFIT equilibrium to the wall, assuming an Eich scaling (Eich et al., PRL 107, 215001 (2011)) of the heat flux layer width. It is planned to add the 3D heat flux model to the HEAT toolset in the future.

A heat flux model based on the guiding center particle drift in vacuum fields has been developed that includes intrinsic and applied 3D magnetic fields as well as scalar electric potentials. For ions, divertor footprints were simulated for multiple kinetic energies and combined based on their respective contribution to the ion’s Maxwellian distribution. For electrons, three different models are considered: a convection model like the one used for ions, a conduction model facilitated by collisions, and a heuristic 3D layer approach. The modeled divertor heat flux pattern on the divertor plates displays the usual striations from the 3D spiral structures (figure), understood to be generated by homoclinic tangles.

Motivated by recent NAS and FESAC reports emphasizing Fusion Pilot Plant development, the OMFIT STEP integrated modeling module is being used to develop compact reactor scenario use cases as part of the AToM SciDAC project. The first step is to develop self-consistent core equilibrium, transport, and stability solutions whose parameters can be used as starting points for more detailed physics and verification studies. Building upon other compact reactor scenario studies, this work is examining performance in a device with Bt = 8 T, R = 4 m, a = 1.4 m, elongation = 2.0, and triangularity = 0.5. Both inductive (Ip = 16 MA, q_95 ~4.9) and steady-state (Ip = 12 MA, q_95 ~ 6.5) scenarios capable of producing 200+ MW of electric power have been identified. These solutions assume only RF-like auxiliary sources which drive current and heat electrons, but provide no core fueling, torque, or ion heating. Although the majority of the net heating is to electrons, the combination of significant core radiated power and strong ion-electron coupling leads to 50% or more of the conducted power being carried by the ions. The largest uncertainty in the core confinement predictions (made using TGLF SAT1) arises from density peaking, with electromagnetic simulations predicting significantly lower levels than electrostatic simulations. Summaries will be presented by C. Holland (UCSD) at the upcoming Asia-Pacific Transport Working Group, EU-US Transport Task Force, and APS-DPP meetings this year.

The paper “Study of H-mode pedestal model for helium plasmas in DIII-D” by Dr. K. Li and coauthors from GA has been accepted for publication in Nuclear Fusion. The paper describes an analysis of pedestal width and height in recent DIII-D H-mode experiments with He plasmas and finds good agreement between the formula for the pedestal width dPsi and the pedestal poloidal beta, dPsi =n 0.095sqrt(beta_p), from experimental observations. The EPED/REPED pedestal model is found to predict well the pedestal structure of the He plasma. It furthermore indicates that the pedestal heights are similar for He and Deuterium plasmas with the same global parameters. The pedestal width of He is approximately 6% larger than that of Deuterium. The study is intended to provide predictions for the ITER non-nuclear Helium phase.